1. Field of the Invention
The invention relates to an additional heating system for a motor vehicle.
2. Background of the Invention
It is known that heating systems can be implemented in motor vehicles by using the heat released by the internal combustion engine. However, the new generation of direct-injection internal combustion engines is significantly more efficient, with the result that the amount of heat produced by these engines is no longer sufficient to heat the interior comfortably in all operating states. Particularly after a cold start, the very long time that elapses before the internal combustion engine produces a sufficient heat output is troublesome.
It is furthermore known that additional heating systems can be implemented by connecting up electric heating elements, such as heating bars.
The underlying object of the invention is therefore to specify an additional heating system for a motor vehicle that can be implemented in a simple manner and nevertheless allows the motor vehicle to be heated up rapidly and reliably.
According to the invention, the object is achieved by virtue of the fact that a heating/air-conditioning unit of the motor vehicle has a compressor, which is driven by an electric motor and is arranged in a refrigerating circuit carrying a refrigerant, the compressor having a switching device for switching from the refrigerating circuit to a heating bypass and back again, the heating bypass diverting the refrigerant that flows at least partially around the electric motor and/or at least the power components of power electronics controlling the electric motor so as to release the absorbed heat generated by the power dissipation of the electric motor and/or the power components to the interior of the vehicle.
The advantage of the invention consists in that the additional heating is achieved by using the compressor/motor unit present in the vehicle. Only a few additional devices have to be installed. The additional heating system according to the invention thus requires no additional installation space in the motor vehicle.
The heating bypass advantageously has a feed duct which carries the refrigerant from an evaporator to a housing containing the electric motor and the compressor, the switching device arranged on the housing being connected to the evaporator by a return duct.
The refrigerant supplied with the waste heat is passed to the evaporator, which heats up the air flowing past, which is directed into the interior of the vehicle. With the use of higher vehicle-system voltages in motor vehicles, air-conditioning compressors that are driven by electric motors and provided with control electronics will soon be standard equipment in vehicles. The only structural measure that has to be taken is to ensure that the output from the compressor is diverted to the evaporator.
In a development of the invention, the electric motor and the compressor are arranged in the housing carrying the power electronics, the refrigerant emerging from the evaporator flowing around the electric motor and/or at least the power components of the engine control electronics before entering the compressor. This has the advantage that the electric motor is cooled by the refrigerant, which is subject to forced circulation.
In a refinement, the refrigerant flows around the electric motor on all sides. Particularly when the electric motor is arranged within a container holding the refrigerant, the heat exchange area that can be achieved between the electric motor and the refrigerant can be considerably increased, it being possible for the heat that arises to be released directly from the electric motor to the refrigerant via its housing. At the same time, the refrigerant surrounding the electric motor acts as a sound absorber, thus significantly reducing the noise that gets through to the outside.
Advantageously at least the power components of the power electronics are arranged in close thermal contact with the refrigerant.
The power components are thus embodied in such a way that they can be cooled by means of a heat exchange surface wetted by the refrigerant. The power electronics are prevented from heating up to an impermissibly high level associated with possible damage to the electronics by the fact that the engine and electronics are cooled by the refrigerant when they reach their limit temperatures. This means that even powerful electric motors that develop a large amount of heat can be used.
In a particularly advantageous development of the invention, the heat exchange surface is embodied as part of the electric motor. In this arrangement, the heat exchange surface comprises a multiplicity of ducts, through which the refrigerant flows, thereby increasing the effective surface area for heat exchange. As a result, the heat can be dissipated by the refrigerant directly in the area in which it arises at the electric motor. The inflow to the heat exchange surface ensures almost complete wetting, and, in particular, it is possible to avoid the formation of stagnation zones or regions of reduced flow velocity, and heat transfer can be improved. As a result, it is possible to achieve further enhanced heat dissipation while simultaneously increasing efficiency.
In a refinement, the power electronics containing the power components are arranged on a thick-film ceramic substrate which is bonded onto the surface of the container by means of a thermally conductive adhesive.
As an alternative, the power electronics containing the power components are arranged on thermally conductive ceramic which is coated on both sides with copper and is soldered onto the heat exchange surface.
In particular, such substrates provide vibration-resistant arrangement of the power electronics in the motor vehicle.
The power electronics advantageously drive the switching device to switch over from the refrigerating circuit to the heating bypass and vice versa. As an alternative, the power electronics are connected to the air-conditioning control unit of the heating/air-conditioning unit, the air-conditioning control unit driving the power electronics to switch the refrigerating circuit over to the heating bypass and vice versa.
In a refinement, the air-conditioning control unit or the power electronics control(s) the electric motor for additional heating as required. If the refrigerant is to be used for its original purpose in the refrigerating circuit to cool the interior of the vehicle, it is passed from the evaporator via the compressor to a condenser and to the throttle and back again to the evaporator. However, if the refrigerant is to be used for additional heating, the refrigerant flowing through the electric motor and the compressor is fed back directly to the evaporator.
When a need is detected, the air-conditioning control unit drives the switching device automatically.
To prevent the engine temperature or the temperature occurring at the power electronics from exceeding a critical limiting value, respective temperature sensors for monitoring these temperatures are integrated into the housing and connected to the air-conditioning control unit and the power electronics to protect the electric motor and the output stages in critical situations.
On the basis of evaluation of the signals supplied by the temperature sensors, the air-conditioning control unit or the power electronics changes or change a control frequency so as to operate the electric motor at an operating point at which a high level of power dissipation can be produced. To increase the heat output, the efficiency of the motor is thus deliberately reduced.
The mechanical power dissipation of the drive unit (e.g. frictional losses of the bearings, compression power) also contributes to an increase in the temperature of the refrigerant.
In a simple embodiment of the invention, the switching device is designed as a valve.